DDS15 — Proper interpretation of Duration_t for negative values

The specification does not fully define the meaning of a Duration_t when the second part is negative.

struct Duration_t {
    long sec;
    unsigned long nanosec;
};

What is he interpretation of a value when sec <0?
Does the nanosec part get interpreted to have the same sign?
If so, then the number between 0 < x < -1 cannot be represented

So the most logical way to interpret a negative value for the sec field appears to be:

value = sec + nanosec 

where sec can be positive or negative but nanosec is always positive.
So the value of negative 1ms would be represented as:

sec = -1
nanosec = 999,000,000

The specification should define the proper interpretation.